1. Field of the Invention
The present invention relates to an electroluminescent device (hereinafter referred to as "EL device") for use in a liquid crystal display device or an image sensor for the purpose of generating illumination light, and to the image sensor which employs such an EL device.
2. Related Background Art
An image processing system such as a facsimile machine, a copying machine, or a computer peripheral device like an image reading apparatus, employs an image sensor for picking up an image or a liquid crystal device for displaying an image.
With electronic office machines widely in use today, there is a growing need for a compact and low cost image sensor or liquid crystal device. One type of image sensor, for example, is the equal magnification image sensor which may be placed in direct contact with an original document in use, without an image formation system included or with an image formation system, if included, having a short optical path.
In a contact type image sensor with so-called selfoc lens, an illumination system and a sensor system are disposed close to each other. In the above quoted full contact image sensor without lens, a document is illuminated through a glass base plate, and thus a sensor system is even more closely disposed to a light source. One of the illumination systems employed in such a device is an LED array. To ensure a uniform illumination level across the full width of the document, an LED array must be compactly arranged. This increases the cost of the image sensor. To achieve a uniform illumination level even with a coarsely arranged array of LED, the illumination system must be placed substantially apart from the document. A desirable compact design of the sensor will not be achieved using such an arrangement. In view of these requirements, a surface light source EL device may be recognized as a promising candidate as a light source for an image sensor.
As shown in FIG. 1, for example, an image sensor with an EL device comprises a transparent plate 1 made of glass or the like, wherein the transparent plate 1 supports a photoelectric transfer section formed of a thin film photosensor array 10 made of amorphous silicon (hereinafter referred to as "a-Si") and the like, storage capacitances, and thin film transistors (not shown). The transparent plate 1 has, on its top portion, a transparent insulating layer 2 serving as a spacer against an original document. An EL device 30 ( not shown ) is disposed beneath the transparent plate 1. The EL device 30 is glued onto the underside of the transparent plate 1 with an adhesive (not shown). As an alternative to the adhesive, a clamp member (not shown) may be employed to secure the EL device 30 onto the transparent plate 1.
The EL device 30 is of a sandwich structure, wherein a metal electrode 32, an insulating layer 33, an EL emission layer 34, an insulating layer 33', and a transparent electrode 35 are stacked in that order from the bottom up onto a substrate 31. A protective film 36 covers the sandwich structure of the EL device 30. A driving signal is applied between the metal electrode 32 and the transparent electrode 35 (i.e., between terminals O and O'), and light is emitted by the EL emission layer 34 sandwiched between the electrodes.
The light emitted by the EL emission layer 34 is transmitted through the transparent electrode 35, the transparent plate 1, and the transparent insulating layer 2, and then reaches a document 100. Light 50, reflected in response to the image on the document 100, enters the photosensor array 10 of the photoelectric transfer section. In response to the incident light, a photoelectric current is generated in the photosensor array 10.
When an EL device constructed as mentioned above is employed as a source of light in an image reading apparatus, the photoelectric transfer section is disposed close to the metal electrode 32 and the transparent electrode 35, both of which serve as the driving electrodes for the EL device. Employed to drive the EL device 30 is a supply voltage with its amplitude ranging from +/-100 V to +/-250 V, its frequency ranging from 50 Hz to 5 kHz, and its waveform as shown in FIG. 2. An electric field appears between the photoelectric transfer section and the EL device 30, becoming a source of noise affecting signals such as a photoelectric current flowing in the photosensor and a voltage of the storage capacitance.
Since the level of the signal derived from the photoelectric current is extremely low, the above described noise affects greatly the output signal from the photoelectric transfer section, thereby presenting a difficulty in accurately picking up a line of image data from the document.
In an attempt to solve the above problem, the inventors of the present invention interposed, between an EL device 30 and a photoelectric transfer section, a transparent conductive layer 40 which is grounded as shown in FIG. 3. This arrangement, however, proved costly.
The same problem arises in a liquid crystal display apparatus, with noise originating in an EL device disturbing a presented image on its display.
Apart from the above problem, the design of an image sensor with an EL device is associated with another problem as described below.
FIG. 4 illustrates a driving circuit for a conventional EL device. In the circuit, the EL device is driven by a self-excitement type inverter which outputs a signal with a constant frequency and a constant voltage to a transformer 9.
Commonly used image sensors employ a CCD type sensor, or a charge storage type sensor made of amorphous silicon or the like. The charge storage type sensor integrates, over a unit time, charge in response to picture information containing light, such as light scattered from the surface of a document, stores in a capacitor a corresponding charge and convert this picture information to a voltage level for processing.
FIG. 5 illustrates a conventional EL device and the circuit of a photoelectric transfer device using a image sensor. FIG. 6 is a cross-sectional view showing roughly this photoelectric transfer device. The image sensor illustrated in FIG. 5 comprises an EL device 30 as a light source, a self-excitement type inverter 20 having a transformer 9, a plurality of storage type photosensors 3 arranged in a row, a sensor driving device 4 and an output signal processing circuit 5.
Storage type photosensors S1-Sn are arrayed, with n representing the total number of photosensors. For example, when n is set up to give a resolution of 8 pixels/mm, a sheet size of A4 includes 1728 bits of information, and a sheet size of B4 includes 2048 bits of information. These photosensors S1-Sn have power supplied by a sensor bias voltage supply 6. As shown in FIG. 6, light emitted alternately by the EL device 30 is directed to a document 100, and scattered by the surface of the document in response to the picture information of the document. The scattered light 50 then enters the storage type photosensors 3.
The pulse supplied by the sensor driving device 4 causes SW1-SWn in the storage photosensors 3 to simultaneously be closed. Throughout this period, each of capacitors C1 through Cn stores each charge corresponding to the integrated value of each scattered light signal entering each of S1 through Sn. After stored charges are converted into voltages, the output signal processing circuit 5 outputs them sequentially to complete a reading operation over a full width of a line.
In the above-described arrangement where a photoelectric transfer device is made of an EL device, a self-excitement type inverter, storage type reading sensors, a sensor driving device and an output signal processing circuit, integrated values of light quantities of the illumination of the document 111-1, 111-2 and 111-3 suffer variations when a storage timing 112 determined by the sensor driving device fluctuates as shown in FIGS. 7(a)-(c), thereby deteriorating the consistency of the sensors' responses.
As illustrated in FIGS. 8(a)-(c), when the EL device is driven by a signal 110' whose frequency is high enough to make insignificant the error caused by fluctuations in the storage timing 112' of the reading sensor, the luminance characteristics of the EL device itself deteriorate rapidly, as expressed by the relationship between the driving frequency of the EL device and the resultant luminance of the EL device as in FIG. 9. Also, high frequency component noises develop, adversely affecting a resulting image. Furthermore, commonly used self-excitement type inverters are bulky in their volume, and contain a costly transformer. This presents a difficulty in implementing a compact and low cost design into an image sensor.